This invention relates to protective grilles and, more particularly, to a novel and highly-effective grille for protecting a loudspeaker or the like, especially one used to signal the existence of an emergency such as fire.
In order to save life and property during a fire or other emergency, it is of the utmost importance to have a reliable audible alarm. Experience has shown that one requirement for reliability is protection of the loudspeaker or other signaling device against weather and vandalism, and fire codes generally require such protection.
Conventionally, sometimes a re-entrant loudspeaker is used to give an alarm: i.e., a loudspeaker that is folded within itself. This folding not only reduces its physical length but also provides a certain measure of protection. In some cases, the speaker cone opens in a direction opposite the direction in which the alarm signal is intended mainly to be propagated, and the sound waves emanating from the speaker cone are reflected from the baffle or reflector mounted in front of the cone and pass around the loudspeaker in the direction of principal propagation. See, for example, the woofer assembly, FIG. 20-57C, The Audio Cyclopedia, Second Edition, 1973, page 1100. In other cases, the horn is doubly folded, and the speaker cone opens in a direction which is the same as the direction in which the alarm signal is intended mainly to be propagated. The sound waves emanating from the speaker cone and passing through an interior horn are then reflected from a baffle or reflector mounted in front of the interior horn, pass around the interior horn in a direction opposite the direction in which the alarm signal is intended mainly to be propagated, and are then reflected by an exterior horn and propagated mainly in the intended direction, which is the same as the direction in which the cone opens. See, for example, the tweeter assembly, FIG. 20-57C, and the exponential folded horn, FIG. 20-57B, The Audio Cyclopedia, 2nd Edition, 1973, page 1100.
Such re-entrant speakers are deficient in a numer of respects. First, they are more expensive than conventional speakers that are not folded. Second, they do not necessarily protect against vandalism, since it is often possible to reach around the speaker cone and damage it.
Accordingly, a grille is sometimes used to protect the speaker cone, and in this case the horn may, but need not, be folded. Grilles are known that permit sound to pass through while excluding rain, vandalizing implements and the like. One example is used in the Model 950 loudspeaker of Federal Sign and Signal Corporation. However, conventional grilles are relatively expensive to manufacture, since it is necessary to construct louvers which lie in separate planes that are perpendicular to the principal propagation axis of the sound waves. Making such louvers in one piece is relatively expensive. Making them in two pieces that can be assembled is often less expensive but is nevertheless costly because separate tooling is required to manufacture the two pieces, since they are different. See, for example, the Model 950 mentioned above and the inner and outer rows of bars of the grille 22 shown in the patent to Walker U.S. Pat. No. 3,306,990 for "Microphone and Speaker Structure for Miniature Receiver and Transmitter", issued Feb. 28, 1967.